Without looking at the file itself, my best guess is that you have a trailing whitespace at the end of the cell, which has the mentioned formatting applied to it.
When inserting new row(s) in table, InDesign copies the paragraph and character style settings (along with any overrides) of the cell over to the new ones it that same column. More specifically, it copies the settings of the last character in the cell, be it a letter, whitespace, or even a break character.
Show Hidden Characters under the
Type menu (it sits at the bottom) or press CMD + ALT + I if you're using Mac or CTRL + ALT + I if you're on a PC. See if there is any symbol at the end of the cell, similar to ones in the screenshot below. There should be only a hash
# symbol at the end of each cell, like in the first cell in the image. If there is something else beside it (or if there's only a symbol other than
#), it is most likely that character which carries the unwanted formatting, which then carries over to new rows.
If you want, you can try selecting it and checking which overrides are applied. Otherwise, you can just clear the overrides. Click on the
Clear overrides button at the bottom of
Paragraph Styles panel, or CMD + click (CTRL + click on PC) to clear only the character overrides, keeping any paragraph overrides.
For a more "permanent" solution, check if there is a cell style applied to a cell. If there is, this style might have a paragraph style selected as default. Either change the cell style or paragraph style settings to your liking.